what type of field is FundNumber? if it's a text field, then enclose it in quotes...
also, you should really use tag... and that may solve the data type issue for the date column, too.
assuming FundMunber is an integer field, try this:
UPDATE tbl_Daily_Nav
SET
[Date] = ,
nav_now = value="#arrNavNow[x]#">,
nav_past = value="#arrNavPast[x]#">,
YTD = value="#arrYTD[x]#">
WHERE FundNumber = value="#arrFundNumber[x]#>
Azadi